For most applications frozen food needs to be brought to the melting temperature before further processing, such as cutting, recipe formulation or cooking can take place. Thawing of frozen food in the household or on an industrial scale is traditionally done by leaving the food material (e.g. at -20 to -30.degree. C.) for a period to thaw in at least ambient temperature. In a factory environment blocks of frozen food e.g. raw fish, meat, poultry, vegetables at -20 to -30.degree. C., are conventionally left in thawing cells wherein the energy is transmitted to the product by convection or conduction.
An alternative to these conventional thawing methods is to thaw the frozen product by microwave energy. The frozen product is placed in a special applicator and subjected to the application of short microwave pulses. The use of pulsed microwave power is dictated by the very low values of the thermal diffusivity of the frozen products and the requirement of long equilibrium times after each pulse in order to avoid excessive heating of parts of the food products. Depending on the size and nature of the frozen product, the processing time when subjected to microwave energy at 2.45 GHz generally ranges from 30 min. to a couple of hours for a 500 to 5000 grams block.
The above discussed thawing methods however suffer from serious drawbacks in that they are relatively slow and that they do not provide a uniform thawing of the food product.